The radio frequency spectrum comprises various radio frequency bands which are respectively allocated to different types of use. Thus, for example, the frequency band around 900 MHz, i.e. between around 872 and 960 MHz, has been allocated to the GSM (for Global System for Mobile communications) telecommunications network.
For some periods, for some locations, it is possible that one or more frequency channels on this band remain unused for the type of use to which they are allocated. Measures carried out by the FCC (Federal Communications Commission) thus show that the use of these frequency bands is not efficient, as it is highly probable, at a certain moment and a given place, that one or more frequency bands of the radio spectrum remain unused.
In order to optimize use of the radio spectrum, it is foreseen to allow, in some conditions, the use of free frequency bands of the radio spectrum for types of use other than those for which they have been allocated. In these conditions, for a given frequency band, the users of the type of use to which this frequency band has been allocated have priority, but are not the only ones to be able to use such a frequency band. Specifically, when such a frequency band is free, i.e. when it is not being used by any user and only until a priority user once again uses it, other users may also be authorized to use it.
In order to use the free frequency bands of the radio spectrum in this way, it is necessary that a terminal is able to modify its transmission parameters depending on the surrounding radio context. Such a terminal implements the principles of cognitive radio.
The FCC has already permitted such a use of the frequency bands that are allocated to television.
However, in order to avoid any disturbance to the use of the frequency bands by the priority users, the other users of these frequency bands must meet certain requirements. Thus it is required that such a non-priority user is designed to be able to detect whether a frequency band of the radio spectrum is free or if it is already being used.
To this end, it has already been proposed to detect the availability of such a frequency band by determining the location of the terminal so as to access a database that is able to provide this terminal with information relating to the state of the radio spectrum in the surroundings of the place it is. However, such detection remains static and requires an adequate network infrastructure.
Detection of whether a frequency band is available has also been provided on the basis of the known characteristics of the telecommunications signal to be detected in the frequency band considered. Detection of this sort has in particular the disadvantage of being based on a priori information relating to the signal which is supposed to be detected and which is potentially transmitted in the frequency band considered.